balanced-parens.js

const isOpening = (s, m) => m[s];
const isClosing = (s, m) => (
  Object.values(m).includes(s)
);
const isValid = (last, s, m) => (
  m[last] !== s
);

const balancedParens = s => {
  const mapping = {
    '(': ')', '[': ']'
  };
  let stack = [];
  let last;
  for (var i = 0; i < s.length; i++) {
    if (isOpening(s[i], mapping)) {
      stack.push(s[i]);
    } else if (isClosing(s[i], mapping)) {
      last = stack.pop();
      if (isValid(last, s[i], mapping)) {
        return false;
      }
    }
  }
  return stack.length === 0 ? true : false;
};

let s = '()';
console.log(balancedParens(s) === true);

s = '(())';
console.log(balancedParens(s) === true);

s = '(';
console.log(balancedParens(s) === false);

s = '(()';
console.log(balancedParens(s) === false);

s = '[';
console.log(balancedParens(s) === false);

s = '[]';
console.log(balancedParens(s) === true);

s = '[)';
console.log(balancedParens(s) === false);

s = '(((]))';
console.log(balancedParens(s) === false);

s = '{ac[bb]}';
console.log(balancedParens(s) === true);